The OPAx241 series and OPAx251 series can drive a wide range of capacitive loads. However, all op amps under certain conditions can be unstable. Op amp configuration, gain, and load value are just a few of the factors to consider when determining stability.

Figure 6-2 and Figure 6-3 show the regions where the OPAx241 series and OPAx251 series have the potential for instability. As shown, the unity gain configuration with low supplies is the most susceptible to the effects of capacitive load. With V_S = 5V, G = 1, and I_OUT = 0, operation remains stable with load capacitance up to approximately 200pF. Increasing a combination of supply voltage, output current, and gain significantly improves capacitive load drive. For example, increasing the supplies to ±15V and gain to 10 drives approximately 2700pF.

Figure 6-4 shows one method to improve capacitive load drive in the unity gain configuration by inserting a resistor inside the feedback loop. This reduces ringing with large capacitive loads while maintaining dc accuracy. For example, with V_S = ±1.35V and R_S = 5kΩ, the OPAx241 series and OPAx251 series perform well with capacitive loads in excess of 1000pF. Without the series resistor, the capacitive load drive is typically 200pF for these conditions. However, this method results in a slight reduction of output voltage swing.

Figure 6-2 Stability—Capacitive Load vs Output Current for Low Supply Voltage

Figure 6-3 Stability—Capacitive Load vs Output Current for ±15V Supplies

Figure 6-4 Series Resistor in Unity Gain Configuration Improves Capacitive Load Drive

Figure 6-5 Low-Side and High-Side Battery Current Sensing